Facebook Developers released the latest version of the comment box social plugin for third-party websites. They added the ability to 'Subscribe' and 'Unsubscribe' specific comments in the comment box. So if your users are interested in a particular part of the conversation, they can easily subscribe by clicking the subscribe button. By clicking the subscribe link, the user will receive a notification when others reply to threads so that they do not miss anything interesting.
This feature will certainly improve usability plugin comments, especially for those who use it as a plugin in their blog comment system. I think the next feature should be the ability to subscribe via RSS or Email. What do you think?